It is well known that the El Niño – Southern Oscillation (ENSO) phenomenon [1]– a quasi periodic oscillation of the coupled ocean atmosphere system over the equatorial Pacific Ocean – conditions the climate of many parts of the world, including southeastern South America. During warm events or El Niño years, with seas surface temperatures in the equatorial Pacific higher than average, precipitation over Uruguay tends to be higher in certain seasons. Conversely, during cold or La Niña events, it tends to rain less [2-3]. In this work, we will represent ENSO by a scalar monthly index that consists of the sea surface temperature anomaly averaged over a box at the central equatorial Pacific called Niño 3.4 region [4]. Observed values of the N3.4 index are available for over a century. The goal of this paper is to show the effect that the ENSO-induced bias in the precipitation has on the cost of energy supply in Uruguay. We further present a methodology to consider N3.4 index in the optimization tools that compute the optimal policy for the use of the water stocked in the hydroelectric subsystem. We present the changes in the operation of the system induced by the consideration of N3.4 index in the optimization process. We focus on the operation of the reservoir of Rincón del Bonete because it is the biggest one in Uruguay. A case study was selected corresponding to the time horizon from August 2009 to July 2010. At the beginning of the selected period the development of an El Niño event was already clear and therefore more rains were expected in the region, in particular for local spring. The evaluation of the results shows the significance of the impact that this foreknowledge has on the operation of the system.